The present invention relates to an interferometric measuring device for measuring the shape especially of rough surfaces of a measured object, having a radiation-producing unit emitting short-coherent radiation, a beam splitter for forming a first and a second beam component, of which the first is directed via an object light path to the measured object and the second is directed via a reference light path to a reflective reference plane, having a superposition element at which the radiation coming from the measured object and the reference plane are brought to superposition, and an image converter which receives the superposed radiation and sends corresponding signals to a device for evaluation, the optical path length of the object light path being changed relative to the optical path length of the reference light path.
Such an interferometric measuring device is known from German DE 197 21 842 C2. In the case of this known measuring device, a radiation-producing unit, such as a light-emitting diode or a superluminescent diode, emits a short-coherent radiation, which is split via a beam splitter into a first beam component guided over an object light path, and a second beam component guided over a reference light path. The reference light path is periodically changed, using two deflector elements and a stationary diffraction grating positioned behind it, by activating the deflector elements, so as to scan the object surface in the depth direction. If the object light path and the reference light path coincide, a maximum interference contrast results, which is detected using an evaluation device post-connected to the photodetector device.
An interferometric measuring device representative of the measuring principle (white-light interferometry or short-coherent interferometry) is also specified in German DE 41 08 944 A1. Here, however, a moved mirror is used to change the light path in the reference ray path. In this method, the surface of the object is imaged on the photodetector device, using an optical system, it being difficult, however, to conduct measurements in cavities.
Additional such interferometric measuring devices and interferometric measuring methods based on white-light interferometry are described by P. de Groot, L. Deck, xe2x80x9cSurface Profiling by Analysis of white-Light Interferograms in the Spatial Frequency Domainxe2x80x9d J. Mod. Opt., Vol. 42, No. 2, 389-401, 1995 and No. T. Maack, G. Notni, W. Schreiber, W.-D. Prenzel, xe2x80x9cEndoskopisches 3-D-Formmesssystemxe2x80x9d, (Endoscopic 3-D Shape Measuring System) in Jahrbuch fxc3xcr Optik und Feinmechanik, Ed. W.-D. Prenzel, Verlag (publisher) Schiele und Schoen, Berlin, 231-240, 1998 verwiesen (submitted).
In the case of the interferometric measuring devices and measuring methods named, one difficulty is making measurements in deep cavities or narrow ducts. One suggestion for a measuring device in which measurements can be performed even in cavities, using white-light interferometry, is shown in German DE 197 21 843 C1. It is proposed there to split a first beam component further into a reference beam component and at least one measuring beam component, an additional beam splitter and the reference mirror being positioned in a common measuring probe. To be sure, such a measuring probe can be introduced into cavities, however, using this device, in each measurement, only a small, dot-like location in the surface can be scanned. In order to take the measure of more locations on the surface in the depth direction, relative motion between measured object and measuring probe is required, an exact lateral coordination, however, being costly and difficult.
The object of the present invention is to make available an interferometric measuring device, of the kind mentioned at the outset, which especially makes possible simplified measurements in deep cavities with great accuracy.
This object is achieved by the features of claim 1. According to this it is provided that an optical probe in the object light path, having an optical device for generating at least one optical intermediate image, be provided.
Similarly to an endoscope or a borescope, in using the optical device, because of the intermediate images, it becomes possible to image the observed surface, besides using high longitudinal resolution, also at high lateral resolution over a path which is long compared to the diameter of the imaging optics. For example, the optical probe can be introduced into the bores of valve seats or into vessels of organisms for the purpose of medical measurements. In contrast to the usual endoscope, quantitative depth information is now obtained. In this connection, an advantageous embodiment is one in which the at least one intermediate image is generated in the object light path. For this, the same optical device is used for illuminating the measured location on the measured object as for transmitting the radiation coming from the measured object to the photodetector device, if it is provided that both the radiation going to the measured object and the radiation coming back from it pass through the optical probe.
The optical image on the photodetector device can be improved by providing, in the reference light path, an equal, further optical probe or at least a glass device for compensating for a glass proportion present in the optical probe with regard to the elements for the intermediate image(s).
A favorable construction, as far as handling is concerned, is one in which the optical motion difference between the first and the second arm is greater than the coherence length of the radiation; the radiation coming from the first mirror and the reflecting element are guided through a common optical probe (common path) using a further radiation portion; in the optical probe, a reference mirror is arranged at such a distance from the measured object that the motion difference between the first mirror and the reflecting element is canceled, and one part of the radiation incident on the reference mirror is reflected to the photodetector device and one part is allowed to pass through to the measured object and is reflected from there to the photodetector device. A further benefit of this design is that the object and reference waves pass through virtually the identical optics assembly, so that aberrations are substantially compensated for. Moreover, this set-up is more resistant to mechanical shocks. In this connection, two embodiment possibilities are for the reference mirror to be provided on a flat face-plate or on a prism.
In this connection, handling can further be simplified by arranging a fiber optic element between the beam splitter and the further beam splitter.
In this design too, splitting essentially into a probe part and a part having a modulation arrangement is realized, handling being also favored.